1. Technical Field
The invention relates to the field of camera systems used for imaging of low light scenes, and more particularly to a reduced in size image intensified camera system.
2. Background Art
Imaging systems for low light level or other low visibility situations are well known and include various types of infrared and image intensifier tube imaging systems.
An image intensifier tube is a vacuum electronics device that is used to amplify light. An image intensifier uses a photocathode to convert the light from an imaged scene into electrons. The electrons are amplified by a thin disc array of millions of channels known as a micro channel plate (MCP). The electrons output from the MCP impinge on a phosphor screen. The phosphor screen is fabricated on one side of a cylindrical fiber optic array. At the opposite end of the array, the resulting amplified image can be viewed. Employment for military uses has resulted in the production of hundreds of thousands of image intensifiers. Typically, these image intensifiers are mass produced in standard industry configurations with the capability of amplifying imagery presented at the 18 mm diameter input. The intensified image is available at the 18 mm diameter image output.
Digital image intensified cameras seek to faithfully transfer the output image produced by an image intensifier tube assembly to a camera chip in order to allow digital dissemination of the intensifier output image. Digital image intensified cameras can be divided into three categories: (1) intensified cameras where the camera chip sensor is directly integrated into the vacuum envelope of the image intensifier, negating the need for a phosphor screen; (2) intensified cameras where the camera chip is physically joined, possibly to a fiber optic taper, to the intensifier external fiber optic output; and, (3) intensified cameras where the image intensifier tube is used as is and an external assembly of intermediate optics between the image intensifier and the camera imaging chip is used relay the image.
U.S. Pat. No. 5,909,309 (“Di Taranto”) by Di Taranto et al. teaches about the use of magnifying and demagnifying optics in an optical train containing an image intensifier as a way to modulate the image size with a design goal in reducing image distortion caused by vignetting. Di Taranto describes an optical train as accommodating various sizes of the image intensifier input and output areas. Commercially available image intensifiers have fixed sized circular active input and outputs capable of handling imagery up to a diameter of 18 mm. Di Taranto gives the same optical subassembly two different designations. These two assemblies are denoted as the format matching assembly and the relay lens assembly. As drawn and described, these two notationally different assemblies are functionally identical in the patent.
Di Taranto's design has several disadvantages. Di Taranto's design takes an existing commercial camera, video camera or camcorder system and inserts two sets of optics and an image intensifier tube between the original camera objective lens and the original camera body. This design essentially uses two relay lens assemblies. One relay assembly, positioned in front of the image intensifier is between the original camera objective lens assembly and the image intensifier. The second relay lens assembly is positioned after the image intensifier. The use of two relay lenses lengthens the optical train and furthermore, a longer optical train makes the entire assembly unwieldy. Di Taranto's patent in configuring a flat focal plane at the output of the image intensifier assembly does not allow for the fact that the high majority of commercially available image intensifiers have concave fiber optic outputs transmitting the intensified imagery. In using a concave fiber optic output, the output of the image intensifier will have a curved image plane. In the Di Taranto patent, while the original camera objective lens will certainly be designed to correct for chromatic aberration, the Di Taranto patent does not consider that the relay lens assembly between the original objective lens and the image intensifier will also have to be designed to correct for chromatic aberration. Di Taranto's design is not practical in that the image intensifier has to be powered. The cylindrical tube containing the image intensifier will have to be punctured to accommodate wires from a battery pack to power the intensifier. Furthermore, the Di Taranto patent is limited to the utilization of image intensification to commercial camera systems with the correspondingly matched, detachable objective lenses. Finally, needless complication is introduced in having to customize the mechanical mechanisms of attaching to different camera systems and customizing both relay lens assemblies to different camera objective lens/body combinations.
U.S. Pat. No. 6,285,018 (the '018 patent) teaches about a device which incorporates a camera chip inside the vacuum envelope. While this device is compact and it does provide a digitized image, it has several drawbacks. The '018 device does not have a microchannel plate (MCP). Because the MCP provides electron amplification, the lack a MCP unfortunately results in having a camera system with less overall optical gain than an image intensifier tube. Furthermore, camera chips that function inside a vacuum are specialty components. These special chips must meet vacuum compatibility requirements that require fabrication with materials that do not outgas contaminates into the internal ultra high vacuum environment of the device. The special nature of these imaging chips limits the rate of imaging improvement because the development costs for higher pixel count sensors have to be averaged over much smaller production quantities. The pixilation of these devices is currently 1600 pixels×1200 pixels, which renders a digital image with less resolution than that of an image intensifier tube.
U.S. Pat. No. 7,015,452 (the '452 patent) teaches about a device with both the microchannel plate and the camera chip inside the vacuum envelope. While the addition of a microchannel plate improves the optical gain of the image intensifier assembly, the supported resolution of the vacuum co-located camera chip is still smaller than that of the MCP and photocathode components. The addition of an MCP and the camera chip in the same envelope also complicates the vacuum processing and sealing of an image intensifier. Prior to the sealing of an image intensifier tube, the intensifier tube components will have been subjected to a high temperature vacuum process. This high temperature vacuum process outgasses the individual internal intensifier components, particularly the MCP with its millions of cylindrical channels in a timely manner. With the addition of a camera chip sensor, the maximum temperature at which the image intensifier assembly can be subjected to is greatly reduced because the maximum temperature that the camera chip can be continuously exposed is around 100 degrees centigrade. Therefore, the processing time is exponentially increased and manufacturing throughput is reduced.
U.S. Pat. No. 7,129,464 teaches a strategy for system compactness in the adoption of a specialty fiber optic taper as the relay optic between the image intensifier and the pixilated camera chip. This taper is in intimate contact with both components. The camera chip in this type of system is also a specialty item. The coverglass or microlens array in material contact with the camera pixel array must be absent in order to enable direct physical contact with the fiber optic taper. Furthermore, not all pixilated camera chips have the removal of the coverglass or microlens array as an available option. This direct optical contact is important for bonding and maximally maintaining image resolution. The custom fiber optic taper matches the image field of the intensifier output to the camera chip format, both of which can be of different lateral dimensions. The image intensifier tube, the custom fiber optic taper, and the pixilated camera chip are glued together. The glue layer by design is as thin as possible to maximize resolution and light transmission. Gluing together all three optical components is an expensive, artisanal process and can be irreversible. Furthermore, the mechanical stability of the glue-glass joint is an issue across large changes in temperature. The permanency of this gluing process makes the upgrading of any individual component expensive and difficult. See, U.S. Pat. No. 7,129,464.
U.S. Pat. No. 6,320,703 (the '703 patent) is an example of a device that uses a spherical wide field of view lens as the input lens for the optical system. One of the optical system possibilities mentioned is an image intensifier based system. The optical train in the '703 patent configures the image intensifier in question to have a fiber optic input. This fiber optic input must also have a spherical concave surface on the input side of the fiber optic. The spherical optic is seated in direct contact with the spherical fiber optic surface in some sort of “shell” optic. As noted in U.S. Pat. No. 8,488,257 there is a disadvantage in optically fusing the entire plane of the two ball lenses.
While the above cited references introduce and disclose a number of noteworthy advances and technological improvements within the art, none completely fulfills the specific objectives achieved by this invention.